Effect of alloying additions on secondary hardening behavior of Mo-containing steels
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I.
INTRODUCTION
SECONDARY hardening steels containing high Co-Ni contents, such as AF1410 (14Co-10Ni-2Cr-1Mo-0.16C)[1] and Aermet 100 (13.4Co-11.1Ni-3.1Cr-1.2Mo-0.23C),[2] were developed for aerospace applications; these steels possess superior toughness even at ultrahigh strength levels and are based on the HY180 (8Co-10Ni-2Cr-1Mo-0.1C).[3,4] The precipitates providing the secondary hardening are fine M2C-type carbides which are formed by the dissolution of M3C-type cementite during aging at temperatures near 500 7C for the high Co-Ni secondary hardening alloy steels. The alloying elements Mo and W form the carbides of M2C type. In contrast, since alloying element Cr alone does not form M2C carbides but M7C3 or M23C6 carbides, hardening with precipitates of the latter carbides could only occur by Cr additions greater than about 9 wt pct.[5,6] However, even the lower Cr contents combined with Mo and/or W additions can contribute to the formation of M2C carbides in which Cr is also dissolved. In addition, the Cr addition seems to be necessary in the high Co-Ni secondary hardening ultrahigh strength alloy steels with superior toughness, such as HY180,[3,4] AF1410,[1] and AerMet 100,[2] even though Cr could weaken the extent of secondary hardening. However, there are few studies on the effect of Cr addition on hardening and fracture behavior. Using a base Fe-10Ni steel, detailed research on the effects of alloying additions on strength and toughness for the HY180 steel, which acted as a starting point for the innovation of high Co-Ni secondary hardening ultrahigh strength steels with the high toughness, was conducted by Speich et al.[3,4] Although Ni has been well known to improve intrinsic toughness of the matrix, i.e., the resistance to the cleavage, its effect on promoting M2C carbides is not known. Recently, research has been reported for the development of the ultrahigh strength secondary hardening steels conH. KWON and J.H. LEE, Professors, and K.B. LEE, Graduate Student, are with the Department of Metallurgy and Materials Engineering, Kookmin University, Seoul 136-702, Korea. Professor Kwon is jointly appointed at the Center for Advanced Aerospace Materials. C.M. KIM, formerly with the Department of Metallurgy and Materials Engineering, College of Engineering, Kookmin University, is with Korea Gas Corporation, Kyunggi-Do 425-150, Korea. H.R. YANG, Professor, is with the Department of Mechanical Engineering, Inchun Junior College, Inchun, 402-750, Korea. Manuscript submitted April 19, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
taining high Co-Ni contents, in which the computer simulation using thermodynamic data was applied to obtain the optimum alloy design for high toughness/strength alloys.[7] While the alloys designed by the compositional modification through the computer simulation exhibited good results in terms of secondary hardening response, the improvement in fracture toughness could not be obtained simply by means of that alloy design. In contrast, the effect of alloying additions on the seco
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